Anodizing Electrolytes Using A Dual Acid System For High Voltage Electrolytic Capacitor Anodes

ABSTRACT

An improved formation electrolyte and method for anodizing valve metal anodes used in electrolytic capacitors, particularly for high voltage sintered tantalum powder anode, is described. The anodizing electrolyte composition is comprised of 1) a phosphorus oxyacid and/or its salt, such as phosphoric acid and ammonium phosphate; 2) a weak inorganic acid/salt (such as boric acid, ammonium borate) or a weak carboxylic acid/salt; 3) water; and 4) a protic solvent or a mixture of two or more protic solvents. The weak mono-carboxylic acid/salt has 2 to 7 carbon atoms and the weak di- or poly-carboxylic acid/salt has 3 to 13 carbon atoms. The present electrolytes have high anodizing breakdown voltage capability and the formed dielectric oxides have improved oxide quality including good oxide hydration resistant ability, and result in more stable capacitor performance. These properties are particularly important for critical applications such as implantable cardioverter defibrillators (ICDs). Significantly, this means that fewer capacitors are needed to meet an ICD&#39;s operating voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Application Ser. No.60/776,168, filed Feb. 23, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to anodizing electrolytes and methods foranodizing valve metal anodes, particularly for high voltage sinteredtantalum powder anodes used in electrolytic capacitors.

2. Prior Art

Electrolytic capacitors are well known for use in a variety ofelectronic equipment such as consumer audio and video equipment, homeappliances, power supplies, industrial electronics, militaryelectronics, computers, telecommunication equipment, entertainmentequipment, automotive devices, lighting ballasts, and implantablemedical devices. In general, electrolytic capacitors comprise an anodeand a cathode segregated from each other by at least one layer ofseparator material impregnated with a working electrolyte. The anode isa valve metal body coated with a layer of the corresponding metal oxideserving as a dielectric.

An implantable cardioverter defibrillator (ICD) operates at highvoltages, typically from about 600 volts to about 800 volts, andrequires high voltage capacitors. Typically, two to four valve metalcapacitors, preferably tantalum capacitors, are connected in series toachieve the desired voltage. Increasing the capacitor working voltagereduces the number of capacitors needed per device. However, developmentof higher voltage capacitors is, in part, limited by the ability of theanodizing electrolyte to form valve metal anodes to a higher voltage.

From time-to-time, the ICD is required to deliver an electrical shocktherapy to the heart to treat tachyarrhythmias, the irregular, rapidheartbeats that can be fatal if left uncorrected. During charging of acapacitor for that purpose, every second is critical for patientsurvival. Rapid charging requires that the capacitors have a stablenon-operating life. Unfortunately, electrolytic capacitors degrade whennot in operation. This so called “shelf” instability is worse inaluminum electrolytic capacitors than in tantalum electrolyticcapacitors. Nonetheless, degradation is mainly due to hydration of thedielectric oxide by water present in the capacitor working electrolyte.Oxide degradation increases the time of the first charging cycle afteran extended non-operation period and reduces the capacitorcharge/discharge energy efficiency. This is undesirable for criticalapplications such as ICDs, in which dielectric oxide degradationincreases capacitor (device) charging time and decreases the useful lifeof the battery or increases battery and device volume.

Only tantalum and aluminum capacitors are currently used in ICDs becausetheir high voltage and high volumetric energy density capabilities.Because ICDs normally operate at from 600 volts to 800 volts and theworking voltage of current tantalum and aluminum capacitors ranges from200 volts to 400 volts, two to four capacitors are used in series forpowering an ICD. However, the development of higher voltage capacitorswould allow fewer capacitors to be used per ICD device.

Correspondingly, the anodes for higher voltage capacitors need to beformed to higher voltages. But, the ability of high voltage anodeformation depends on a number of factors including tantalum powdermicro-morphology, powder chemistry, press/sintering conditions,anodizing electrolyte composition, and anodizing protocols. Electrolytecomposition, especially the anions (phosphate in case of phosphoric acidand phosphate salts) in electrolytes is a critical factor for successfulhigh voltage anode formation. The phosphoric/polyglycol-based anodizingelectrolytes have been used in forming high voltage tantalum anodes.This type of electrolyte consists of phosphoric/phosphate solutes andglycol or polyglycol solvents.

It is observed that the anodic oxide formed in phosphate-basedelectrolytes contains significant phosphorus incorporated therein. Ithas been proved that phosphorous improves dielectric oxide hydrationresistance and long-term performance stability of the resultingcapacitor. However, phosphoric/phosphate solutes have relatively loweranodizing breakdown voltages than some of the weak inorganic (e.g. boricacid and borate) or carboxylic acids. Although lowering the electrolyteconductivity by reducing the phosphoric/phosphate concentration orincreasing the solvent content can enhance the anodizing breakdownvoltage, the electrolyte eventually becomes impractical when its IR dropbecomes too high due to increased electrolyte resistivity.

Certain inorganic acids/salts and weak organic acids/salts have higherformation voltage capability. But, the anodic oxides formed in theseelectrolyte compositions do not have the oxide hydration resistance andlong-term performance stability characteristic ofphosphoric/phosphate-based electrolytes.

Therefore, an improved anodizing electrolyte and method of anodizationis needed to not only provide for higher voltage anodic oxide formation,but also to improve DC leakage and hydration resistance properties. Thepresent invention provides a mixed anodizing electrolyte composition aswell as an improved anodizing method that provide high voltage oxideformation capability while still maintaining low DC leakage andhydration resistance ability.

SUMMARY OF THE INVENTION

In that respect, this invention has two objectives. The first is toprovide an anodizing electrolyte comprised of: 1) a phosphorus oxyacidand/or its salt, such as phosphoric acid and ammonium phosphate; 2) aweaker inorganic acid/salt (such as boric acid, ammonium borate) or aweak mono-carboxylic acid/salt having 2 to 7 carbons or a weak di- orpoly-carboxylic acid having 3 to 13 carbons; 3) water; and 4) a proticsolvent or a mixture of two or more protic solvents. The secondobjective is to provide an improved anodizing method for valve metalstructures in an anodizing electrolyte containing phosphorusoxyacids/salts and a weaker inorganic acid/salt (such as boric acid,ammonium borate) or a weak mono-carboxylic acid/salt having 2 to 7carbons or a weak di- or poly-carboxylic acid having 3 to 13 carbons.

The electrolytes have relatively high anodizing breakdown voltages,which allows for formation of high voltage dielectric oxides, andresults in improved dielectric oxide quality. This means that theresulting capacitor has higher operating voltage, lower DC leakage, goodhydration resistance and more stable lifetime performance. Theseproperties are particularly important for critical applications such asrequired by implantable cardioverter defibrillators (ICDs). When used inICDs, capacitors made with anodes formed according to the presentinvention, which can be formed to higher formation voltages, allow forfewer capacitors per device, provide shorter first charging time, higherenergy efficiency, and more stable lifetime performance.

The foregoing and additional objects, advantages, and characterizingfeatures of the present invention will become increasingly more apparentupon a reading of the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the present invention is focused on sintered tantalum powderanodes, it applies to all valve metals used in electrolytic capacitorssuch as, but not limited to, tantalum, vanadium, niobium, aluminum,titanium, zirconium, hafnium, and mixtures thereof, in the form of foil(etched or unetched), sintered powder bodies, and like porousstructures.

The dielectric oxide on a valve metal in an electrolytic capacitor isnormally formed using a technique known as anodizing. Passing an anodiccurrent through a valve metal immersed in an anodizing (formation)electrolyte does this. The thickness of the resulting anodic oxide isproportional to the anodizing voltage. The desired oxide thickness isdetermined by the capacitor rated voltage and other required properties.For a given dielectric oxide thickness, the volumetric capacitance andenergy density of a capacitor are functions of the specific surface areaof the valve metal anode. To increase capacitor volumetric energydensity, porous valve metal structures are normally used. Examplesinclude an etched aluminum foil for an aluminum capacitor and a pressedand sintered tantalum powder body for a tantalum capacitor. Furthermore,the dielectric oxide quality depends on a number of factors includingthe type and purity of the valve metal, anode micromorphology, anodesize and geometry, electrolyte composition, temperature, and anodizingprotocols.

In that light, this invention is directed to anodizing (formation)electrolytes and methods of anodization. The composition of an anodizingelectrolyte affects anodizing breakdown voltage and the quality of theformed anodic oxide. An anodic oxide normally contains speciesincorporated from the anodizing electrolyte during dielectric oxideformation. For example, the outer layer of an anodic oxide formed inphosphoric acid/phosphate-containing electrolytes contains phosphorus.The amount of the incorporated phosphorous depends on the concentrationof the phosphoric acid/phosphate in the anodizing electrolytes,temperature, and anodizing current density. Although the incorporatedphosphorus slightly decreases the dielectric constant, it benefits thequality of the anodic oxide by lowering DC leakage and improving oxidehydration resistance. Improved hydration resistance helps stabilize theshelf life and long-term performance of the resulting capacitor.Phosphate-based electrolytes, however, have limited anodizing breakdownvoltage and are not capable of formation of relatively high voltageanodic oxides.

Certain weaker inorganic acids (e.g., boric acid) and carboxylic acids(e.g., acetic acid, adipic acid, azelaic acid, dodecanedioic acid) havehigher anodizing breakdown voltage capability than phosphoricacid/phosphate electrolytes, but the anodic oxide formed in them hashigher DC leakage, poor hydration resistance ability, and less stablelong-term performance. Boric acid and large carboxylic acids also havelimited solubility, especially at lower temperatures.

However, the present invention describes an anodizing electrolytecomposition comprised of: 1) a phosphorus oxyacid and/or its salt, suchas phosphoric acid and ammonium phosphate; 2) a weak inorganic acid/saltor a weak carboxylic acid/salt; 3) water; and 4) a protic solvent or amixture of two or more protic solvents. An improved anodizing method foranodizing a valve metal structure in an electrolyte containingphosphorus oxyacids/salts and a weaker inorganic acid/salt (such asboric acid, ammonium borate) or a weak mono-carboxylic acid/salt having2 to 7 carbon atoms or a weak di- or poly-carboxylic acid having 3 to 13carbon atoms is also described.

In particular, the mixed acid electrolytes comprise, by weight, fromabout 5% to about 80% water, up to about 90% of a protic solvent, fromabout 0.1% to about 15% phosphorus acid or its salts, and from about0.5% to about 15% of a weak inorganic or organic carboxylic acid, ortheir mixture, and their salts. The anodizing electrolyte preferably hasa conductivity of about 20 μS to about 10,000 μS, more preferably from100 μS to about 1,000 μS, at 40° C.

The solvent is a protic solvent including one selected from the groupconsisting of alkylene glycols (examples include, but are not limitedto, ethylene glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, propylene glycol, trimethylene glycol, dipropyleneglycol, glycerol, 2-methyl-1,3-propanediol, 1,4-butanediol,2,3-butanediol, 2,2-dimethyl-1,3-propanediol, 2,4-pentanediol,2,5-hexanediol, and combinations thereof), polyalkylene glycols(examples include, but are not limited to, polyethylene glycols,polypropylene glycols, polyethylenepropylene glycol copolymers, andmixtures thereof), and alkylene or polyalkylene glycol monoethers(examples include, but are not limited to, ethylene glycol methyl ether,ethylene glycol ethyl ether, ethylene glycol butyl ether, diethyleneglycol ethyl ether, diethylene glycol methyl ether, diethylene glycolbutyl ether, dipropylene glycol methyl ether, tripropylene glycol methylether, and mixtures thereof). The solvent preferably has a molecularweight of less than 1000.

The phosphorus oxyacids/salts include, but are not limited to,phosphoric acid, ammonium dihydrogen phosphate, ammonium hydrogenphosphate, sodium phosphates, potassium phosphates, hypophosphoric acid,phosphorous acid, hypophosphorous acid, metaphosphoric acids, andmixtures thereof.

The weak inorganic acids/salts include, but are not limited to, boricacid, sodium borate, ammonium borate, ammonium tetraborate, ammoniumpentaborate, and mixtures thereof.

The carboxylic acids include mono-, di-, and poly-carboxylic acids, andmixtures thereof. The mono-carboxylic acids have from 2 to 7 carbons,and have a straight or branched chain or cyclic structures. Examples ofmono-carboxylic acids/salts include, but are not limited to, aceticacid, propanoic acid, butyric acid, iso-butyric acid, trimethyl aceticacid, cyclohexanecarboxylic acid, dicyclohexylacetic acid, and mixturesthereof.

The di-carboxylic acids have from 3 to 13 carbons, and have a straightor branched chain or cyclic structure. Examples of di-carboxylicacids/salts include, but are not limited to, malonic acid, glutaricacid, adipic acid, pimelic acid, azelaic acid, brassylic acid,dodecanedioic acid, 1,2-cyclohexanedicarboxylic acid,cyclohexyldicarboxylic acid, methylmalonic acid, dimethylmalonic acid,2,2-dimethylsuccinic acid, 2-methylglutaric acid, 2,2-dimethylglutaricacid, 3-(tert-butyl)adipic acid, and mixtures thereof.

Examples of poly-carboxylic acids/salts include, but are not limited to,citric acid, tartaric acid, 1,3,5-cyclohexanetricarboxylic acid, andmixtures thereof.

Ammonium salts of the acids discussed above can be produced in situ byreacting the corresponding acid with ammonium hydroxide solution orammonia gas.

The following example describes the manner and process of making andusing an anodizing electrolyte according to the present invention, andit sets forth the best mode contemplated by the inventors of carryingout the invention, but it is not to be construed as limiting.

EXAMPLE

Tantalum powders suitable for use as an anode for an electrolyticcapacitor are commercially available in two main types. Sodium reductiontantalum powders are available from H.C. Starck Inc., Newton, Mass.under the “NH” family designation. Beam melt tantalum powders areavailable from H.C. Starck Inc., Newton, Mass. under the “QR” familydescription. However, the present invention is not intended to belimited to these types of tantalum powders. Instead, the presentinvention is applicable to all types of valve metals and, in particular,all types of tantalum, whether in powder form or otherwise.

The anodizing breakdown voltage values of sintered tantalum pellets inseveral present invention electrolytes containing a mixture of water,polyethylene glycol 400, phosphoric acid and one of a group of a secondacid (boric acid, acetic acid, adipic acid, azelaic acid, pimelic acid,2,2-dimethylglutaric acid, trimethylacetic acid) were tested. Inparticular, the following electrolyte formulations were used:

The formation electrolytes consisted of: about 88% PEG 400 in water,about 0.8M of one of the second acids and the conductivity was adjustedto 120 μS/cm by phosphoric acid. In particular, a pre-mix of 88% PEG and12% de-ionized water was made. Then 0.8M of each of the second acids wasadded and the conductivity was adjusted with H₃PO₄ to 120 μS/cm.

The anodes were individually formed in a jacketed beaker using2.5-watt/gram protocols similar to that described in described in U.S.Pat. No. 6,231,993 to Stephenson et al., which is assigned to theassignee of the present invention and incorporated herein by reference.

The current is initially set in a range of about 5 mA/gram to about 100mA/gram amount of valve metal material during the formation process ofeither a sodium reduction or beam melt pressed valve metal powderstructure. The formation voltage is then raised in intervals of about 10volts to about 100 volts between periods when the formation voltage isturned off. The formation voltage is turned off for at least 10 minutesto allow for cooling and replenishment inside the anode pellet by fresh,relatively cool electrolyte having a conductivity more closely matchingthat of the anodization bath. It should also be pointed out that thecurrent for each formation step preferably becomes less and less as theformation voltage increases. This is in order to keep the wattage frombeing too high. The formation protocol used in this example is a“constant wattage” protocol. The wattage increases with time at constantcurrent within each formation step, but the maximum wattage for allformation steps is kept constant throughout the entire formationprocess. This requires the current to be reduced for each formation stepas the formation voltage increases. The voltage intervals for eachformation step preferably become less and less as the formation voltagegets closer to the target formation voltage. This is because as theformation voltage approaches the target formation voltage, the oxidecoating on the anode pellet becomes thicker, which reduces heatdissipation and permeability of the pellet. Therefore, the voltageintervals are preferably decreased.

The formation protocol for a sodium reduced tantalum powder pellet wasas follows. In this example, the pellet had a weight of about 8.3 gramsand the target formation voltage was 500 volts. For each acid, threeanodes were formed.

1. The power supply was turned on with a peak current of 279 mA to 75volts. The power supply was then turned off for about half an hour.

2. The power supply was turned back on with a peak of 182 mA to 115volts. The power supply was then turned off for about half an hour.

3. The power supply was turned back on with a peak of 144 mA to 145volts. The power supply was then turned off for about half an hour.

4. The power supply was turned back on with a peak of 120 mA to 175volts. The power supply was then turned off for about half an hour.

5. The power supply was turned back on with a peak of 105 mA to 200volts. The power supply was then turned off for about half an hour.

6. The power supply was turned back on with a peak of 95 mA to 220volts. The power supply was then turned off for about half an hour.

7. The power supply was turned back on with a peak of 87 mA to 240volts. The power supply was then turned off for about half an hour.

8. The power supply was turned back on with a peak of 81 mA to 260volts. The power supply was then turned off for about half an hour.

9. The power supply was turned back on with a peak of 75 mA to 280volts. The power supply was then turned off for about half an hour.

10. The power supply was turned back on with a peak of 70 mA to 300volts. The power supply was then turned off for about half an hour.

11. The power supply was turned back on with a peak of 65 mA to 320volts. The power supply was then turned off for about half an hour.

12. The power supply was turned back on with a peak of 62 mA to 340volts. The power supply was then turned off for about half an hour.

13. The power supply was turned back on with a peak of 58 mA to 360volts. The power supply was then turned off for about half an hour.

14. The power supply was turned back on with a peak of 55 mA to 380volts. The power supply was then turned off for about half an hour.

15. The power supply was turned back on with a peak of 52 mA to 400volts. The power supply was then turned off for about half an hour.

16. The power supply was turned back on with a peak of 50 mA to 420volts. The power supply was then turned off for about half an hour.

17. The power supply was turned back on with a peak of 48 mA to 440volts. The power supply was then turned off for about half an hour.

18. The power supply was turned back on with a peak of 46 mA to 460volts. The power supply was then turned off for about half an hour.

19. The power supply was turned back on with a peak of 44 mA to 480volts. The power supply was then turned off for about half an hour.

20. The power supply was turned back on with a peak of 42 mA to 500volts. The power supply was then turned off for about half an hour.

The anodized pellet was then rinsed and dried. This was followed by heattreat and reformation steps.

The formation protocol for a beam melt (BM) tantalum powder pellet wasas follows. In this example, the pellet had a weight of about 4.4 gramsand the desired target formation voltage was 500 volts. Three anodeswere formed.

1. The power supply was turned on with a peak current of 147 mA to 75volts. The power supply was then turned off for about half an hour.

2. The power supply was turned back on with a peak of 96 mA to 115volts. The power supply was then turned off for about half an hour.

3. The power supply was turned back on with a peak of 76 mA to 145volts. The power supply was then turned off for about half an hour.

4. The power supply was turned back on with a peak of 63 mA to 175volts. The power supply was then turned off for about half an hour.

5. The power supply was turned back on with a peak of 55 mA to 200volts. The power supply was then turned off for about half an hour.

6. The power supply was turned back on with a peak of 50 mA to 220volts. The power supply was then turned off for about half an hour.

7. The power supply was turned back on with a peak of 46 mA to 240volts. The power supply was then turned off for about half an hour.

8. The power supply was turned back on with a peak of 42 mA to 260volts. The power supply was then turned off for about half an hour.

9. The power supply was turned back on with a peak of 39 mA to 280volts. The power supply was then turned off for about half an hour.

10. The power supply was turned back on with a peak of 37 mA to 300volts. The power supply was then turned off for about half an hour.

11. The power supply was turned back on with a peak of 34 mA to 320volts. The power supply was then turned off for about half an hour.

12. The power supply was turned back on with a peak of 32 mA to 340volts. The power supply was then turned off for about half an hour.

13. The power supply was turned back on with a peak of 31 mA to 360volts. The power supply was then turned off for about half an hour.

14. The power supply was turned back on with a peak of 29 mA to 380volts. The power supply was then turned off for about half an hour.

15. The power supply was turned back on with a peak of 28 mA to 400volts. The power supply was then turned off for about half an hour.

16. The power supply was turned back on with a peak of 26 mA to 420volts. The power supply was then turned off for about half an hour.

17. The power supply was turned back on with a peak of 25 mA to 440volts. The power supply was then turned off for about half an hour.

18. The power supply was turned back on with a peak of 24 mA to 460volts. The power supply was then turned off for about half an hour.

19. The power supply was turned back on with a peak of 23 mA to 480volts. The power supply was then turned off for about half an hour.

20. The power supply was turned back on with a peak of 22 mA to 500volts. The power supply was then turned off for about half an hour.

The anodized pellet was then rinsed and dried. This was followed by heattreat and reformation steps.

If desired, the formation process for either a sodium reduction or beammelt tantalum powder can be periodically interrupted and the anodizedpellet subjected to a heat treatment step. This consists of removing theanode pellet from the anodization electrolyte bath. The anode pellet isthen rinsed and dried followed by heat treatment according to thearticle by D. M. Smyth et al., “Heat-Treatment of Anodic Oxide Films onTantalum”, Journal of the Electrochemical Society, vol. 110, No. 12, pp.1264-1271, December 1963. This publication is incorporated herein byreference.

Table 1 shows that the control electrolyte that contained onlyphosphoric acid broke down at 340V on the beam melt powder pellet and265V on the sodium reduce powder pellet. However, all of the secondacids either had comparable or higher breakdown voltages. Boric acid hadthe highest breakdown voltage among the acids tested. TABLE 1 FormationBreakdown Voltage, V BM powder Na reduced 2^(nd) Acid pellet powderpellet Phosphoric acid 340 265 Acetic Acid 376 329 Boric Acid 500 329Azelaic Acid 440 296 Pimelic Acid 337 325 2,2-Dimethylglutaric acid 419308 Trimethylacetic acid 345 298

Thus, the electrolytes of the present invention have high anodizingbreakdown voltages and allow for higher voltage dielectric oxideformation. These properties are strongly desired for criticalapplications such as implantable cardioverter defibrillators (ICDs).When using in the ICDs, the capacitors made with anodes formed accordingto the present invention provide short first pulse charging time, highenergy efficiency, and stable lifetime performance.

It is appreciated that various modifications to the present inventiveconcepts described herein may be apparent to those of ordinary skill inthe art without departing from the spirit and scope of the presentinvention as defined by the herein appended claims.

1. An anodizing electrolyte for providing a dielectric oxide on a valvemetal, the anodizing electrolyte comprising: a) water; b) a phosphorusoxyacid or salt thereof; and c) at least one of the group consisting ofan inorganic acid, a salt of the inorganic acid, a carboxylic acid, asalt of the carboxylic acid, and mixtures thereof.
 2. The anodizingelectrolyte of claim 1 wherein the water is present in a range of fromabout 5% to about 80%, by weight.
 3. The anodizing electrolyte of claim1 wherein the phosphorus oxyacid is in a range of from about 0.1% toabout 15%, by weight.
 4. The anodizing electrolyte of claim 1 whereinthe phosphorus oxyacid is selected from the group consisting ofphosphoric acid, hypophosphoric acid, phosphorous acid, hypophosphorousacid, metaphosphoric acids, and mixtures thereof.
 5. The anodizingelectrolyte of claim 1 wherein the phosphorus oxyacid salt is selectedfrom the group consisting of ammonium dihydrogen phosphate, ammoniumhydrogen phosphate, sodium phosphates, potassium phosphates, andmixtures thereof.
 6. The anodizing electrolyte of claim 1 wherein theinorganic acid or the carboxylic acid is in a range of from about 0.5%to about 15%, by weight.
 7. The anodizing electrolyte of claim 1 whereinthe inorganic acid is boric acid and the inorganic acid salt is selectedfrom the group consisting of ammonium borate, sodium borate, ammoniumtetraborate, ammonium pentaborate, and mixtures thereof.
 8. Theanodizing electrolyte of claim 1 wherein the carboxylic acid is amono-carboxylic acid having from 2 to 7 carbon atoms.
 9. The anodizingelectrolyte of claim 1 wherein the carboxylic acid is a di-carboxylicacid or a poly-carboxylic acid having from 3 to 13 carbon atoms.
 10. Theanodizing electrolyte of claim 1 wherein the carboxylic acid is selectedfrom the group consisting of acetic acid, propanoic acid, butyric acid,iso-butyric acid, trimethyl acetic acid, cyclohexanecarboxylic acid,dicyclohexylacetic acid, malonic acid, glutaric acid, adipic acid,pimelic acid, azelaic acid, brassylic acid, dodecanedioic acid,1,2-cyclohexanedicarboxylic acid, cyclohexyldicarboxylic acid,methylmalonic acid, dimethylmalonic acid, 2,2-dimethylsuccinic acid,2-methylglutaric acid, 2,2-dimethylglutaric acid, 3-(tert-butyl)adipicacid, citric acid, tartaric acid, 1,3,5-cyclohexanetricarboxylic acid,and mixtures thereof.
 11. The anodizing electrolyte of claim 1 whereinthe valve metal is selected from the group consisting of tantalum,vanadium, niobium, aluminum, titanium, zirconium, hafnium, and mixturesthereof.
 12. The anodizing electrolyte of claim 1 wherein theelectrolyte further includes a protic solvent.
 13. The anodizingelectrolyte of claim 12 wherein the protic solvent is present in at upto about 90%, by weight.
 14. The anodizing electrolyte of claim 12wherein the protic solvent is selected from the group consisting ofalkylene glycols, polyalkylene glycols, alkylene glycol monoethers,polyalkylene glycol monoethers, and mixtures thereof.
 15. The anodizingelectrolyte of claim 12 wherein the protic solvent is selected from thegroup consisting of ethylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol, propylene glycol, trimethylene glycol,dipropylene glycol, glycerol, 2-methyl-1,3-propanediol, 1,4-butanediol,2,3-butanediol, 2,2-dimethyl-1,3-propanediol, 2,4-pentanediol,2,5-hexanediol, polyethylene glycols, polypropylene glycols,polyethylenepropylene glycol copolymers, ethylene glycol methyl ether,ethylene glycol ethyl ether, ethylene glycol butyl ether, diethyleneglycol ethyl ether, diethylene glycol methyl ether, diethylene glycolbutyl ether, dipropylene glycol methyl ether, tripropylene glycol methylether, and mixtures thereof).
 16. The anodizing electrolyte of claim 1having a conductivity of about 20 μS to about 10,000 μS at 40° C. 17.The anodizing electrolyte of claim 1 having a conductivity of about 100μS to about 1,000 μS at 40° C.
 18. An anodizing electrolyte forproviding a dielectric oxide on a valve metal, the anodizing electrolyteconsisting essentially of: a) water; b) a phosphorus oxyacid; and c) asecond acid selected from the group consisting of boric acid, aceticacid, propanoic acid, butyric acid, iso-butyric acid, trimethyl aceticacid, cyclohexanecarboxylic acid, dicyclohexylacetic acid, malonic acid,glutaric acid, adipic acid, pimelic acid, azelaic acid, brassylic acid,dodecanedioic acid, 1,2-cyclohexanedicarboxylic acid,cyclohexyldicarboxylic acid, methylmalonic acid, dimethylmalonic acid,2,2-dimethylsuccinic acid, 2-methylglutaric acid, 2,2-dimethylglutaricacid, 3-(tert-butyl)adipic acid, citric acid, tartaric acid,1,3,5-cyclohexanetricarboxylic acid, and mixtures thereof.
 19. Theanodizing electrolyte of claim 18 wherein the water is present in arange of from about 5% to about 80%, the phosphorus oxyacid is in arange of from about 0.1% to about 15%, and the second acid is in a rangeof from about 0.5% to about 15%, by weight.
 20. The anodizingelectrolyte of claim 18 having a conductivity of about 20 μS to about10,000 μS at 40° C.
 21. A method for anodizing a valve metal structure,comprising the steps of: a) providing the valve metal structure selectedfrom the group consisting of tantalum, vanadium, niobium, aluminum,titanium, zirconium, hafnium, and mixtures thereof; b) providing ananodizing electrolyte comprising water, a phosphorus oxyacid, and asecond acid selected from the group consisting of boric acid, aceticacid, propanoic acid, butyric acid, iso-butyric acid, trimethyl aceticacid, cyclohexanecarboxylic acid, dicyclohexylacetic acid, malonic acid,glutaric acid, adipic acid, pimelic acid, azelaic acid, brassylic acid,dodecanedioic acid, 1,2-cyclohexanedicarboxylic acid,cyclohexyldicarboxylic acid, methylmalonic acid, dimethylmalonic acid,2,2-dimethylsuccinic acid, 2-methylglutaric acid, 2,2-dimethylglutaricacid, 3-(tert-butyl)adipic acid, citric acid, tartaric acid,1,3,5-cyclohexanetricarboxylic acid, and mixtures thereof; and c)applying a current to the valve metal immersed in the anodizingelectrolyte until a target formation voltage is reached.
 22. The methodof claim 21 including providing the valve metal structure being selectedfrom the group consisting of an etched foil, an unetched foil, and asintered powder body.
 23. The method of claim 21 including periodicallyturning off the formation current and letting the valve metal structurerest in the anodizing electrolyte.
 24. The method of claim 23 includingturning off the formation current for at least 10 minutes.
 25. Themethod of claim 21 including providing the current in a range of fromabout 5 mA/gram to about 100 mA/gram amount of the valve metal.
 26. Themethod of claim 21 including raising a formation voltage in intervals offrom about 10 volts to about 100 volts toward the target formationvoltage between periodically turning off the formation current andletting the valve metal structure rest in the anodizing electrolyte. 27.The method of claim 21 including anodizing the valve metal structureusing a constant wattage protocol where wattage increases with time atconstant current within each formation step between when the formationcurrent is turned off, but the maximum wattage for each formation stepis kept constant throughout the entire formation process.
 28. The methodof claim 21 including providing the anodizing electrolyte having thewater in a range of from about 5% to about 80%, the phosphorus oxyacidin a range of from about 0.1% to about 15%, and the second acid in arange of from about 0.5% to about 15%, by weight.
 29. The method ofclaim 21 including providing the electrolyte having a conductivity ofabout 20 μS to about 10,000 μS at 40° C.